\documentclass{article}
\usepackage{catoptions}
\makeatletter
% The star (*) variant executes code #1 within the group. #1 must be 
% executable in this case. The un-starred version simply takes the 
% material outside the group.
\robust@def*\aftergroupexecute{\cpt@testst\cpt@aftergroupexecute}
\robust@def\cpt@aftergroupexecute#1{%
	\ifboolTF{cpt@st}{#1}{}%
	% \ag@elt is local to current group:
	\def\ag@elt{aftergroup@execute@\romannumeral\currentgrouplevel}%
	\ifcsndefTF\ag@elt{}{%
		\aftercsname\aftergroup\ag@elt
		% for post \aftergroup resetting:
		\csn@xdef\ag@elt{\gundefcsn{\ag@elt}}%
	}%
	\csn@xdef\ag@elt{\expandcsnonce\ag@elt\unexpanded{#1}}%
}
\makeatother
% Tests
\begin{document}
\def\x{FOO}
\def\y{ZOO}
\begingroup
	\begingroup
		\hbox{%
			\vbox to 5cm{%
					$
						\aftergroupexecute{\def\x{foo}}%
						\aftergroupexecute{\def\y{zoo}}%
					$
					%\show\x
					\show\y
				}%
			\show\x
		}%
		\show\x
	\endgroup
	\show\x
\endgroup
\show\x
\end{document}
